Alzheimer's disease

Alzheimer's disease is a brain illness that slowly damages memory, thinking and, over time, the ability to do everyday tasks. It is the most common cause of dementia. It is behind about 60 to 80 out of every 100 cases.^1,8

About 7.4 million Americans age 65 and older live with Alzheimer's in 2026. That is about 1 in 9 people in that age group.² Alzheimer's is not a normal part of growing older.⁸ Learning how it works and how it changes can help you plan. It can also help you get good care.

What happens in the brain

Alzheimer's involves two main changes:^1,8

• Amyloid plaques. Amyloid (a sticky protein) clumps together in the spaces between nerve cells.
• Tau tangles. Tau (a protein that normally helps hold a nerve cell's shape) twists into tangles inside the cells.

Over time, nerve cells lose their links to each other and die. The brain shrinks.¹ The damage usually starts in parts of the brain that store new memories. One is the hippocampus (a small, curved area deep in the brain). This is why trouble remembering new things is often the first sign.^1,8

Most people develop a few plaques and tangles as they age. People with Alzheimer's develop many more, and in a pattern that spreads through the brain.⁸

Alzheimer's often does not travel alone. Other brain changes are common in the same person, such as Lewy body disease or LATE. They can add to symptoms.³ When more than one cause is at work, it is called mixed dementia.

Early onset and late onset

Genes that cause the disease. Changes in three genes, called APP, PSEN1 and PSEN2, almost always lead to Alzheimer's before 65. A child of a parent with one of these changes has a 50-50 chance of inheriting it. These genes explain only 10 to 15 out of 100 early-onset cases.⁶

APOE, a risk gene. Everyone has the APOE gene. Its e4 form raises the risk of Alzheimer's. About 15 to 25 out of 100 people carry one copy. About 2 to 5 out of 100 carry two copies. Its e2 form may lower risk. But APOE cannot predict for sure who will get Alzheimer's.⁶ The test matters most when someone is thinking about anti-amyloid drugs.¹⁴ Read more on genes and dementia.

Down syndrome. People with Down syndrome have an extra copy of chromosome 21, which carries the APP gene. Half or more of them develop Alzheimer's, often in their 50s or 60s.^1,6 See Down syndrome and Alzheimer's.

Younger people face extra challenges, such as work, raising children and money. See Social Security disability for young-onset dementia.

Symptoms by stage

Alzheimer's changes slowly, over years. Each person moves at their own pace. Our stages of dementia section covers each stage in depth.

Before symptoms (preclinical). Brain changes are building, but the person thinks and acts normally.¹ See preclinical Alzheimer's.

Mild cognitive impairment (MCI). Memory or thinking problems are more than normal for age. The person still manages daily life on their own.¹ Not everyone with MCI has Alzheimer's. See mild cognitive impairment.

Early stage (mild dementia). Many people still drive, work and see friends.⁷ Common signs include:^1,7

• Forgetting new names or things just read, and asking the same question again
• Losing things more often
• Trouble with planning, money or paying bills
• Taking longer to do daily tasks
• Mood or personality changes

This is the best time to make legal, money and health care plans while the person can take part.⁷ See early-stage dementia.

Middle stage (moderate dementia). This is usually the longest stage. It can last many years.⁷ The person may forget parts of their own life story. They may be confused about where they are or what day it is. They may need help choosing clothes. Sleep problems, wandering, bladder accidents, suspicion of others and repeated actions are common.⁷ Some people see or believe things that are not real.¹ See middle-stage dementia.

Late stage (severe dementia). The person needs care around the clock. They lose the ability to hold a conversation, and later to walk, sit up or swallow safely. Infections like pneumonia become more likely.⁷ Music, gentle touch and a familiar voice can still bring comfort.⁷ See late-stage dementia and hospice care.

The new biology-based definition (2024)

In June 2024, an Alzheimer's Association workgroup published new rules for diagnosing and staging Alzheimer's.^3,4 The big change is in how the disease is defined.

Defined by biology, not symptoms alone. The workgroup says Alzheimer's begins in the brain before any symptoms. A doctor confirms it with biomarkers. These are measurable signs of disease, such as proteins in blood or spinal fluid, or changes on a brain scan. Symptoms alone are not enough, because other illnesses can cause the same problems.^3,4

Two groups of biomarker tests.³

• Core 1 tests turn abnormal early. They include an amyloid PET scan, approved spinal fluid tests, and accurate blood tests, especially one called p-tau217. One abnormal Core 1 result is enough to diagnose Alzheimer's.
• Core 2 tests turn abnormal later. They include a tau PET scan and some other fluid tests. They help show how far the disease has gone and how likely it is to be causing symptoms.

Staging. The criteria combine two kinds of stages.³ A biological stage, from A to D, shows how far tau has spread on a tau PET scan. A clinical stage describes symptoms, from none, to MCI, to mild, moderate and severe dementia.

What the criteria do not say. They are not a step-by-step guide for doctors. The group advises against testing people with no memory or thinking problems outside research studies. One reason is that no treatment is approved for people without symptoms.^3,4

Not every expert agrees. In late 2024, an International Working Group of experts disagreed. They said people with abnormal biomarkers but normal thinking should be called "at risk," not told they have Alzheimer's. Most such people will not get symptoms in the near future, they noted, and the label could cause harm.⁵

What this means for you: if you have no symptoms, a positive test does not mean dementia is coming soon. Ask your doctor before getting tested, and read preclinical Alzheimer's.

How Alzheimer's is diagnosed

A doctor looks at the whole picture:¹

• Talks with the person and a family member about health, medicines and daily life
• Gives memory and thinking tests. See memory and thinking tests.
• Orders blood and urine tests to look for other causes. Some causes are treatable, such as medicine side effects, infections or sleep problems. See treatable conditions that look like dementia.
• Checks for depression
• Orders a brain scan (CT, MRI or PET), a spinal fluid test or a blood test for Alzheimer's proteins

Blood tests are new. In May 2025, the FDA cleared the first blood test that helps doctors find amyloid. It is for adults 55 and older with memory symptoms who are seen at specialty clinics. It is not a screening test and should not be used alone.¹⁰ An Alzheimer's Association guideline from 2025 says specialists may use accurate blood tests for people who already have thinking problems. Some blood tests are good at ruling Alzheimer's out. A positive result from those tests still needs a PET scan or spinal fluid test to confirm it.¹¹ See blood tests for Alzheimer's, brain scans and which specialist to see.

Treatment overview (as of October 2026)

There is no cure. But treatment can help, and early diagnosis opens more choices, including clinical trials.¹ See the full dementia medicines section.

How the newer drugs are given. Lecanemab is an IV infusion every two weeks. It can also be a weekly shot under the skin, given at home.^14,12 Donanemab is an IV infusion every four weeks. It was studied with treatment stopped once plaques cleared.^16,13 Both carry the FDA's strongest warning, a boxed warning, about ARIA. An APOE gene test is advised before starting.^12,13,14

Paying for them. Medicare Part B can cover the IV forms. Your doctor must take part in an approved registry. You usually pay 20% after the deductible, plus scans and visits.¹⁵ See paying for new Alzheimer's drugs and are anti-amyloid treatments right for us?

Living well and lowering risk

Good daily care can make every stage easier. Steady routines, enjoyable activity, time with others and a safe home all help. See caring for someone with dementia, meaningful activities and making the home safer.

Healthy habits may lower the risk of memory loss or slow it. Two examples are staying active and caring for your heart.⁹ Learn more about blood pressure and the brain, exercise for brain health and hearing and vision.

Caregivers need support too. Nearly 13 million Americans give unpaid care to someone with dementia.² See caregiver stress and burnout.

When to get help

See a doctor if you or someone close to you has memory or thinking changes that affect daily life. Trouble with bills, getting lost in a familiar place, or new mood changes are reasons to check.⁹ Some causes can be treated.¹ Bring a family member and a list of medicines. See the first doctor visit about memory.

The Alzheimer's Association 24/7 Helpline, 800-272-3900, can answer questions and point you to local help.

Education only. This page is general information written from the sources listed. It is not medical, legal or financial advice and does not replace a doctor, therapist or lawyer who knows your situation. How we write and check pages.